Helium radiography with a digital tracking calorimeter—a Monte Carlo study for secondary track rejection

Abstract

Radiation therapy using protons and heavier ions is a fast-growing therapeutic option for cancer patients. A clinical system for particle imaging in particle therapy would enable online patient position verification, estimation of the dose deposition through range monitoring and a reduction of uncertainties in the calculation of the relative stopping power of the patient.Several prototype imaging modalities offer radiography and computed tomography using protons and heavy ions. A Digital Tracking Calorimeter (DTC), currently under development, has been proposed as one such detector. In the DTC 43 longitudinal layers of laterally stacked ALPIDE CMOS monolithic active pixel sensor chips are able to reconstruct a large number of simultaneously recorded proton tracks.In this study, we explored the capability of the DTC for helium imaging which offers favorable spatial resolution over proton imaging. Helium ions exhibit a larger cross section for inelastic nuclear interactions, increasing the number of produced secondaries in the imaged object and in the detector itself. To that end, a filtering process able to remove a large fraction of the secondaries was identified, and the track reconstruction process was adapted for helium ions.By filtering on the energy loss along the tracks, on the incoming angle and on the particle ranges, 97.5% of the secondaries were removed. After passing through 16 cm water, 50.0% of the primary helium ions survived; after the proposed filtering 42.4% of the primaries remained; finally after subsequent image reconstruction 31% of the primaries remained. Helium track reconstruction leads to more track matching errors compared to protons due to the increased available focus strength of the helium beam. In a head phantom radiograph, the Water Equivalent

Experimental helium‐beam radiography with a high‐energy beam: Water‐equivalent thickness calibration and first image‐quality results

“…The precision values from other studies are at maximum 40% better than the measured mean WET precision of 1.55% of this work. Considering the dE measurement of this work is performed within 300𝜇m silicon, while the residual energy and/or range is measured in detection systems with thicknesses above 250 mm, 23,25 the observed WET precision is surprisingly close to the best precision achieved until now. Furthermore, the comparison shows that an extension of the WET range toward WETs below 280.58 mm should not be performed using the same initial energy, since the WET precision deteriorates quickly.…”

“…For a simulation study about helium‐ion imaging with a tracking calorimeter, which also includes higher WET ranges up to ∼310 mm, a simulated helium‐ion precision as low as 0.9% for the same WET range of (280.86 and 299.8 mm) can be extracted 25 …”

“…Currently, it is a common practice to use the WET distribution of single ions instead of a WET distribution of pixel values to assess the precision. This is because the single-ion distribution is not dependent on pixel size nor on imaging dose, 23,25,56 which enables an easy and direct comparison of different detection systems or methods. In this work, the energy deposition of each Heion after having crossed a homogeneous phantom is converted to a WET value using the established calibration curve.…”

“…For a simulation study about helium-ion imaging with a tracking calorimeter, which also includes higher WET ranges up to ∼310 mm, a simulated helium-ion precision as low as 0.9% for the same WET range of (280.86 and 299.8 mm) can be extracted. 25 For a completely fair comparison, the simulations and assumptions have to be experimentally verified. However, it can give a first indication on how the measured WET precision values compare to a state-of -the-art and a future imaging system.…”

“…Moreover, undesired inelastic nuclear interactions within the detection system are often reduced in comparison to residual energy/range detectors with thicknesses of more than 250 mm. [23][24][25] This applies to situations in which low-WET regions are imaged using a residual range/energy detector and a high initial energy. The deeper an ion travels into a range detector (i.e., the higher the residual energy of the ion downstream of the object to be imaged), the more likely fragmentation and loss of primary ions is to occur in the detection system, which is not the case for a thin dE detector layer.…”